摘要:

When low‐frequency underwater sound “reflects” from the Arctic ice cover, not only will it be reflected from the plane, and specularly scattered from roughness elements, but it will also be diffracted at leads and reradiated from flexural waves in the ice. In the case of near‐grazing forward scatter from closely spaced roughness elements there may also be a coherent forward scattered boundary wave detectable near the under‐ice surface. These phenomena have been studied in an anechoic tank by pulse transmission from an underwater point source to a large floating acrylic model of the Arctic ice. Scaling of the flexural wave velocityB(ωh)1/2at frequency ω and ice thicknesshis accurately achieved becauseB, for the acrylic model, is well within the range of typical values in arctic ice. Scaling of the sound scattered from roughness elements is also accomplished in terms of frequency and the length dimension. Consequently, the laboratory model at frequency 50 kHz represents 100‐Hz sound encountering a typical Arctic canopy which is112 mthick and about a kilometer in extent. The physical contributors to the gross “reflection coefficient” are studied for models of a plane ice layer, an Arctic ice ridge, edges of leads, and a rubble field of ice. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2023581

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A strong “refraction branch diffraction” has been observed on ocean bottom hydrophone data from the Rivera Ocean Seismic Experiment (ROSE). Modeling of the interaction between seismic/acoustic energy and the ocean bottom has shown that this arrival could be caused by scattering from common seafloor features. Seismic/acoustic wave propagation was computed for models of marine seismic refraction lines using a 2D heterogeneous formulation of the elastic wave equation. These models demonstrate that a significant amount of energy is diffracted from seafloor structures such as hills and valleys when both the direct water wave and the refracted compressional wave (traveling through the upper crust) are incident upon a structure. Since much of the diffracted energy travels through the crust, the diffracted arrivals appear on the refraction branch of the seismograms at large ranges. Energy partitioning at the seafloor of the two incident wave types produce both compressional (P) and shear (S) diffracted phases in the upper oceanic crust. The large models used also clearly demonstrate the existence of phases which are theoretically possible but rarely identified in marine seismic data such as the pseudo‐Rayleigh wave and thePandSinterference head waves.

ISSN:0001-4966    

DOI:10.1121/1.2023584

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The accuracy of the Kirchhoff approximation for randomly rough, pressure‐release surfaces has been examined through comparison with exact numerical results based on solving an integral equation. A Gaussian roughness spectrum was chosen for the surfaces, and a Monte Carlo procedure was used to obtain the average bistatic scattering cross section. Results will be presented that illustrate the breakdown of the Kirchhoff approximation as the incident grazing angle is reduced and as the average radius of curvature is reduced. The accuracy of shadowing corrections to the Kirchhoff approximation will also be discussed. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2023589

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Improved holographic interference patterns of the vibrational modes of the front plate of a p′i‐p′a will be shown. Most of the modes lie below 2000 Hz and those having larger amplitudes of vibration lie between 320 Hz to 1300 Hz. Vibrations of the front plate due to plucking of the strings have been studied. It is found experimentally that any one of the three strongest plate modes can be excited when the fundamental frequency of the plucked string is equal to or is a subharmonic of the frequency of that mode. The p′i‐p′a sound generated by plucking the string is picked up by a sound level meter and the Fourier components are obtained by using a digital storage oscilloscope coupled to an Apple II computer. Because of the lack of an anechoic chamber, the sound level meter was placed not more than 1 m from the front plate. Even with this limitation, it was discovered that the sound pressure levels associated with different mode patterns decreased, with increasing distance, at different rates. This seems to mean that listeners at different distances from the player would hear music of different tone quality. Holographic vibration patterns and frequency response curves were also obtained for another Chinese musical instrument, the cheng. Efforts were made to identify various wood or air resonances. The cheng is a much larger instrument than p′i‐p′a, and both its front and back plates can vibrate strongly.

ISSN:0001-4966    

DOI:10.1121/1.2023593

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

In order to examine the limitations associated with this technique, a systematic study of the various parameters affecting intensity measurements to determine sound power will be presented. The effect of the finite approximation, phase mismatch, inter‐active intensity, the number of points, and the spatial distribution of the points will be discussed, keeping in mind actual measurement conditions. Somein‐situexperimental studies will be presented. For a typical section of a textile machine, the power measured via ISO 3744 in a semianechoic field compared well with that measured in a semireverberant room using intensity, confirming the validity of the technique. Further, it will be shown that intensity can be used to differentiate the noise emitted by the frame from that of the spindle. A systematic way of including a criterion of precision will be proposed. This leads to a tolerance which gives the degree of confidence on the measurement and which points out clearly unfavorable conditions.

ISSN:0001-4966    

DOI:10.1121/1.2023598

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The problem of the localization and characterization of sound sources is tackled, for the case of a set of monopole sources, starting with the analytic expressions describing the field in combination with a gradient method for the optimization process [M. Boxet al.,Nonlinear Optimization Techniques(Olivier&Boyd, London)]. To locate the sources three measurement variables are used: the 3D vectorial active intensity, the normal active intensity, and the quadratic mean pressure. Simulations for three to five emitting sources, aimed at predicting their positions and spectral densities are made. The minimum number of measurement positions that lead to convergence of the algorithm is investigated. The agreement between prediction and reality is good, even when the sources are coherent. In parallel, the impact of the choice of measurement variable on the precision of the results is studied. In the very near field of the source, it is found that identification via 3D intensimetry requires a significantly lower number of measurement positions than does that via normal intensity or quadratic mean pressure.

ISSN:0001-4966    

DOI:10.1121/1.2023599

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Last year, the discovery of SACER inN2/H2mixtures was reported [J. Acoust. Soc. Am. Suppl. 178, S45 (1985)]. This paper discribes similar measurements inN2/HeandN2/CH4mixtures. Sound amplification has been observed, and its dependence upon gas pressure, sound frequency, foreign gas concentration, and vibrational temperature is that generally expected from a crude relaxation model for the gases and published values of the relaxation times. However, as in the case withN2/H2mixtures, the gain is an order of magnitude bigger and persists longer than expected. An electrical discharge was passed through the gas to initiate the acoustical oscillations and elevate the vibrational temperature. InN2/CH4mixtures, the gain and the nature of the electrical discharge were found to depend on the number of times the discharge was passed through the gas. This result is interpreted as being due to a chemical reaction betweenN2andCH4that produces an impurity that shortened the relaxation time. [Work sponsored by the ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2023611

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A hybrid model was developed for the analysis of constrained‐layer damping whereby the base plate is described by exact elasticity theory, while the elastomer and constraining layers are analogous to Kerwin's model [J. Acoust. Soc. Am.31, 952–962 (1959)]. Under fluid loading, the success of the technique is measured by a damping efficiency, the ratio of loss in the elastomer to the total loss due to viscoelastic and radiation damping. This efficiency is usually evaluated by combining the viscoelastic loss from a composite plate in vacuum, for antisymmetric and symmetric waves separately with the radiation loss from a single fluid‐loaded plate. This assumes simple addition of the two effects for a fluid‐loaded constrained plate. Extension of the hybrid model to include fluid loading shows that at higher frequencies the presence of the two types of waves redistributes the energy across the plate, invalidating the presumed additive property. Thus it proves necessary to compute damping efficiency from analysis of the fluid‐loaded composite plate with inclusion of both antisymmetric and symmetric waves. [Work supported by the ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2023620

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Extra‐high‐frequency audiometry may have a clinical use as an indicator of the traumatic effects of noise. The purpose of this study was to determine if cardiovascular endurance has a relationship to the extra‐high‐frequency thresholds of hearing. Industrial workers (ages 27–30, with normal hearing in the conventional frequency region) exposed to daily noise doses of 85–88 dBA served as subjects. Cardiovascular endurance and extra‐high‐frequency thresholds were determined for 50 subjects. A two‐factor ANOVA (Group × Frequency) revealed no significant differences in the extra‐high‐frequency thresholds of two groups of industrial workers with varying cardiovascular endurance. For 50 subjects, no significant correlations existed between the thresholds of hearing at the extra‐high frequencies and cardiovascular endurance. It was concluded that cardiovascular endurance cannot be used to determine individual susceptability to the noxious effects of noise.

ISSN:0001-4966    

DOI:10.1121/1.2023626

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

As a follow‐on to an idea presented earlier [G. R. Price, J. Acoust. Soc. Am.56, 195–197 (1974)] an integrated mathematical model of the external, middle, and inner ears has been developed for the cat ear. It allows the calculation of stapes displacements (among other values) in response to any free‐field sound pressure. An absolute limit to displacement of about 20 μ peak to peak is assumed, based on the dimensions of the annular ligament and in accord with the measurements of Guinan and Peake [J. Acoust Soc. Am.41, 1237–1261 (1967)]. Where hazard from intense impulses, such as gunfire, are concerned, the effect of this limitation on stapes displacement is profound, producing a spectral bias (higher velocities being transmitted at high frequencies) as well as the blocking of transmission into the cochlea when amplitudes exceed the limiting values. This action may in part explain why low‐frequency impulses are much less hazardous than their energy content would seem to indicate as well as why hearing losses are not more severe for very intense sounds (150 dB and up) commonly found around weapons.

ISSN:0001-4966    

DOI:10.1121/1.2023628

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP